This invention relates to monitoring particulate matter in fluid flow streams circulated in boiler/steam cycle process equipment.
A long-standing challenge for those engaged in power production is that of minimizing metal corrosion and transport in boiler/steam cycle processes. As is well known, water is extremely corrosive at high temperatures and oxidizes the walls of boilers, heat exchangers and associated piping as it circulates through them, forming soluble and suspended oxides of iron and copper. Importantly, a small buildup of these metal oxides can reduce the heat transfer rates sufficiently in a boiler/steam system to cause a catastrophic failure.
While the potential for corrosion transport related to upsets in system chemistry in boiler/steam cycle process equipment is widely understood, many basic questions remain. Some investigators have even suggested that velocity changes—which affect primarily the distribution of insoluble particulates—may play a more important role in the boiler cycle than does a steady corrosion rate. Questions have also been raised as to what extent conventional chemical measurements, i.e., conductivity, pH, dissolved oxygen, and specific ion measurements, indicate the level of metal transport in a system and whether anti-corrosion additives interact at all with insoluble contaminants. Uncertainties abound in part because previous monitoring efforts did not provide for correlating particulate iron and copper concentrations with an exact time of transport.
Moreover, only recently, after years of testing at various power plants, has the magnitude of the component contributed by insoluble particulates to corrosion transport become apparent. Using corrosion transport sampling equipment augmented by ion-exchange technology, investigators have now deduced that typically between 90 and 95 percent of the specii involved in metal transport phenomena are particulates large enough to be captured on a 0.45 micron filter. Obscuring this finding earlier was the fact that significant variations in the relative percentages of soluble and suspended metal oxides exist between individual power plants. Furthermore, these relative percentages vary with:                Sample stream and location sampled.        Oxidation/reduction environment at a particular sampling location.        Temperature, flow rate and system design.        System chemistry.        
Substantially larger than the specii present at the onset of particulate formation, the insoluble particles measured with this augmented corrosion transport sampling equipment were, for each sample stream tested, first removed from it with the use of a 0.45 micron filter pad; and then the sample stream was directed through a series of ion-exchange resins, located downstream of the filter pad. The ion-exchange resins were used to selectively adsorb soluble species such as iron and copper ions. After the sample stream had been pushed through the pad and resins for a relatively long preset time interval (typically 24 hours), both the pad and resins were removed and separately dissolved, digested in acid and then ultimately subjected to a sophisticated and time-consuming atomic absorption spectroscopy-based metals analysis, so that their respective metal contents could be measured.
Unfortunately, flow-altering events—which typically occur during startup, shutdown and other non-steady state conditions—tend to occur over much shorter time intervals than a full 24-hour period. As a consequence, the impact of such events cannot be readily isolated from prior art data based primarily on 24-hour “composite” samples and the like.
For testing system particulate iron under non-steady state conditions, little has changed until now since boiler manufacturers Babcock and Wilcox introduced, many years ago, comparison charts which enabled users, who first captured particulate iron from one liter samples on filters, to estimate the type of iron oxide and the approximate concentration of particulate iron present based upon the appearance (color and intensity or darkness of color) of the material captured on the filter. This method, still used today during power plant startups, provides a quick, but nonetheless fairly accurate, test of system particulate iron. One of the drawbacks of this filter method is that low level trace metal analysis determinations are not possible.
Promising alternate approaches have involved using automated samplers to collect discrete (“grab”) samples from a fluid flow stream either at periodic intervals or only whenever major excursions or “spikes” in the levels of contaminants in a flow stream are detected. The former approach has proven itself to be largely impractical, except possibly in the case of peaking/cycling boiler/steam systems. Recent extended monitoring of boiler/steam units having minimal starts during a year, for example, indicates that few, if any, distinct and significant flow-altering events occur in them for weeks at a time. Collecting samples only during major excursions, on the other hand, requires the use of an instrument for detecting particulates, such as the particle monitor employed in the automated sampler combination disclosed by Bryant and Veal in U.S. Pat. No. 5,798,699, or a particle counter.
Distinctly different from the particle monitor, the particle counter, which is based on the optical method of light blocking or “extinction”, can detect 2 micron or larger particles in fluids and size them, putting them into “bins” of defined size ranges; the particle monitor, by contrast, can at most track trends in particulate concentrations—specifically, concentrations in aggregate of particles of the order of 1 micron or larger in size. Nevertheless, now that 0.45 micron and larger suspended particles are known to dominate metal transport to such a high degree, the particle monitor has gained wider acceptance as the instrument of choice for detecting particulates. Formerly, that position, as is well known, was held by the turbidimeter, an instrument which, unlike either the particle monitor or the particle counter, can, under favorable conditions, detect the onset of particulate formation.
Whether the particle monitor or the particle counter is used for detecting particulates, major obstacles remain for those seeking to isolate the impact of flow-altering events on corrosion transport. Both of these alternate approaches depend upon collecting individual discrete samples which tend to age quickly. And as they age, information is readily lost regarding the relative percentages of soluble and insoluble contaminants which may be present in the flow stream. Further, analyses of metal oxide transport phenomena are complicated by the fact that the quantity of particles of a given size range detected by the particle counter is dependent upon, among other things, the composition of the particles and the mixture of which they are a part. Specifically, recent testing by the applicants has shown that the particle count may be substantially lower than what would be expected on the basis of the mass of metal oxides captured on a 0.45 micron filter.